High power audio amplifier having feedback provided by a winding cofilar with an output transformer primary winding

ABSTRACT

A high-power audio amplifier employing both cathode and anode load unity coupled cofilarly wound transformer primary windings, providing negative feedback by means of a transformer winding which is cofilar with respect to the output windings and have the same number of turns. Individual tube bias controls are employed for all output tubes.

United States Patent Inventor Mioliub R. Nestorovic Endwell, N.Y.

Appl. No' 799,867

Filed Feb. 17, 1969 Patented Apr. 6, 1971 Assignee McIntosh Laboratory, Inc.

Binghamton, N.Y.

HIGH POWER AUDIO AMPLIFIER HAVING FEEDBACK PROVIDED BY A WINDING CO-FILAR WITH AN OUTPUT TRANSFORMER PRIMARY Primary Examiner-Roy Lake Assistant Examiner-James B. Mullins Attorney-Hurvitz, Rose and Greene WINDING 14 Claims, 2 Drawing Figs.

U.S.Cl 330/82, AB TRA T: A high-power audio amplifier employing both 330/99, 330/119, 330/122 cathode and anode load unity coupled cofilarly wound trans- Int. Cl H03t1/36, former primary winding pr ing n gative feedback by H03f 3/28 means of a transformer winding which is cofilar with respect Field of Search 330/81, 82, to the output windings and have the same number of turns. In-

98, 1 19, 99, 122, 123 dividual tube bias controls are employed for all output tubes.

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2 sheetwsheet 2 WS wwa i Maw HILWM Q INVENTSJR wouua RNESTDROVIC ATTORNEYS HIGH POWER AUDIO AMPLIFIER HAVING FEEDBACK PROVIDED BY A WINDING CO-FILAR WITH AN OUT- PUT TRANSFORMER PRIMARY WINDING BACKGROUND OF THE INVENTION The amount of negative feedback which can be applied in a power amplifier using an output transformer is limited at high frequencies by the leakage inductance between the primary and feedback windings of the output transformer, since this introduces phase shift. Feedback is usually taken from a secondary winding, or from a tertiary winding closely coupled with a secondary winding. On the latter basis, the maximum amount of stable feedback obtainable is about 25 db. The actual amount attainable may be less, depending on the quality of the output transformer and the parameters of the amplifier circuitry.

The amount of feedback attainable may be increased by deriving feedback from the primary winding. But, in so doing, second harmonic distortion is increased, because the current flowing through the primary winding causes a voltage drop across the resistance of the primary winding, which is high in second harmonic distortion. This is not reduced in proportion to amount of feedback. 1

The present invention solves the problem of increasing feedback with consequent decrease of all forms of distortion, by employing a feedback winding cofilarly wound with the primary and having the same number of turns, and therefore having the same phase and voltage as the primary, but carrying essentially no current. The distortion caused by current flow in the primary winding resistance no longer affects feedback voltage. It is then possible by means of the present invention to utilize up to 40 db. of negative feedback, with excellent stability resulting in extremely low nonlinear distortion.

The amplifier of the invention employs four pentodes connected in parallel on either side of the push-pull unity coupled circuit. These pentodes are normally not quite identical, therefore DC balance circuitry is provided for each of the tubes. This circuitry is unusual in that the drive stage is directly coupled to each of the output tubes and at the same time allows independent bias adjustment for each of the output tubes.

SUMMARY OF THE INVENTION A wideband audio unity coupled pentode power amplifier, having very low distortion, attained by negative feedback applied by a primary filar winding having the same number of turns as the primary winding, and having unity coupled plate, cathode and screen grid primary windings. This invention represents an improvement over the U.S. Pat. to Frank McIntosh No. 2,646,467 issued Jul. 21, i953, and the US. Pat. to Frank McIntosh No. 2,477,074, issued Jul. 26, 1949.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1 and 1A comprise a schematic circuit diagram of an amplifier according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to drawing 1A, consider first the output stage. Triodes T, and T are push-pull driven cathode follower drivers, having their anodes directly connected to 470 v. terminal 10. The cathode of T, is connected for AC through audio bypass capacitor C, and a resistance R to a primary winding PW.,, which in turn has a center tap 12, connected to l65 v. Across C,, are connected in parallel four potentiometers D,, D D D The net DC voltage appearing at the cathode of T, is 42 v. No audio signal appears across I),, D D D because of C but a DC voltage of 22 v. does appear, and any part of this can be taken off by sliders to provide adjustable bias, separately, for pentodes P,, P P P respectively. Duplicate circuitry applies to T and to pentodes P P,,, P-,, P,,, which provide the alternate halves of a push-pull pair, operating class AB.

The plates of P,, P P P are all connected directly in parallel and to one side of primary winding PW,, and the plates of P P,,, inclusive, are similarly connected to the other side of PW,. The latter has a center tap at 470 v.

The screen grids of P,P are supplied with v. from center tap 14 of a screen grid push-pull primary winding PW cofilarly related turn for turn with winding PW, and otherwise not coupled thereto. The cathodes of P,P,, are connected to primary winding PW the center tap 15 of which is connected essentially to ground, 1.2 v. volts DC appearing at the end terminals of PW It follows that the cathode and the screen grid of each of P,P,,, inclusive, are always at the same AC voltage, although at different DC voltages. This is per se well known. See the McIntosh patents. PW, is cofilarly wound with PW, and also its end terminals are coupled to the end terminals of PW, by coupling capacitors C C which have negligible impedance at the audio frequencies of interest. The plates winding PW, by virtue of cross connection of its end terminals to P,--P and to P,-,P,,, via leads 16, 17, maintains the cathodes of P,.P 4 at the same AC voltage as the anodes of P,-,-P,,, and the cathodes of P P,, at the same AC voltage as the anodes of P,P.,. This latter feature represents the essence of the classic McIntosh amplifier.

AC signal to the control grids of P,P,, occurs via cathodes of the tubes T, and T which are bootstrapped loaded by winding PW filarly wound with PW, and having the same number of turns. The end terminals of PW, are connected to the grids of P,P., through R and potentiometers D,-D.,. An identical arrangement is provided for the other side of the push-pull array. The DC biases on the control grids are individually adjusted by potentiometers D,D,,, using a milliammeter, not illustrated, which can be connected in turn parallel to the cathode resistors (R R R R R R R R of the output tubes P,P,,. Thereby it assures that the DC, no signal current is divided evenly on each tube.

A fifth, feedback winding is provided, PW which is filarly wound with PW, and has the same number of turns as PW, and which supplies its voltage via leads 19, 20, in an external feedback loop, now to be described.

The triodes T,, T, are driven by triodes T T respectively. The latter are anode loaded, and capacitively coupled to the grids of T,, T,, and constitute a bootstrapped balanced driver stage. The anode of T is connected via resistance R and one side of winding PW, to a positive supply 470 v. via center tap 10. The anode of T is connected via resistance R and the other side of winding PW, to center tap 10. The elements in the cathode circuit of T and T resistors R,,, R R and bypass capacitors C and C not only set up the correct bias for the operation of T and T, but in connection with PW provide a local balanced unity coupled feedback loop.

T and T are connected as a balanced cathode follower and drive the grids of T and T,,. The resistors R R R and symmetrically R R and R form a voltage dividing network to set the grid bias of T and T,,. The large capacitors C,, and C, bypass the resistors R and R respectfully at audio frequencies.

T T form a differential amplifier with the anodes directly coupled to the grids of T T The phase inverter composed of triodes T,,, T,,,, is provided with input signal from single ended isolation cathode follower stage T,,. One side of PW, drives the grid of the triode T,,. Therefore, we have a differential amplifier, used here as a phase inverter, in which one side (T is driven with the signal from the cathode of the T,, while the other side (T,,,) is driven by the overall Unity Coupled Nega- OPERATION Reviewing now in somewhat more detail the operation of the present system, and commencing now at the input end of the amplifier, the input signal enters the amplifier either via the front panel input jacks or via the input jacks, 15, on the rear panel of the unit. All input jacks are in parallel. Capacitor C,, in conjunction with the volume control R fonn a 6 db. per octave, subsonic filter, which can be disconnected by switch 5,.

One-half of the twin triode T, is used, and this half is connected as a cathode follower; the second section is not used. The primary function of this stage is to isolate the input from the feedback loop, thereby resulting in no change in distortion at higher frequencies and a better square wave response independent of the setting of the volume control R T T are connected to form a differential amplifier which acts as a phase inverter. The signal is capacitively coupled from the input cathode follower T through C to the grid of T,. The overall Unity Coupled Negative Feedback (32 db.) is fed onto the grid of T using the elements R R C, and R,. as a dividing network. The elements R C and C are arranged to feed a portion of the feedback voltage to the plate of T,. This local feedback loop is incorporated to give a better balance of AC voltages at the plates of the differential amplifier for frequencies up to 100 kHz.

The zener diode D, (10 volts) in series with diode D is used to protect the cathodes of T,, T against a potential exceeding the cathode-filament insulation limits. This is required since upon initial turn on, before T T begin conduction, the potential at the cathodes of T T would be -l65 v. if the diodes D, and D, were not incorporated. With the addition of the diodes D, and D,, at initial turn on, the Zener diode conducts and a potential of l v. is present on the cathodes of T and T As the tubes begin conducting, the potential on the cathodes reaches +0.9 v. and the diodes are no longer conductive. Resistors R and R provide the cathode current path to the l65 v. supply.

The signal output of T,, T, is directly coupled to the grids of T T,,, a differential amplifier. Any AC unbalance present in the signal from T,, 'l is reduced to less than 1 percent at the plates of T-,, T,,. The high value cathode resistors R and R, are used to allow DC self balance for T T The capacitor C acts as an AC short circuit between the cathodes causing the complete circuit to act as a differential amplifier.

From the plates of T T the signal is then directly coupled to the grids of T T., which forms a balanced cathode follower. This circuitry is used to provide a low impedance source for feeding the signal to T T The elements R24, C and R, on one side and symmetrically R C and R,-, on the other side form a filter which adjusts the phase characteristic at low frequencies for stable operation of the amplifier. At the same time, the resistors R and R on one side and symmetrically R and R,-, on the other side act as a DC voltage dividing network to apply the proper bias voltage to the grids of T T The signal is next directly coupled to the twin triode T T,, which is a bootstrapped balanced driver stage. The plate load resistors for this stage R and R are bootstrapped to the output transformer plate primary winding, thus allowing very high stage gain and large output voltage swing. The driver stage tubes T T have on their cathodes a local balanced Unity Coupled Negative Feedback loop derived from a winding which is unity coupled with the primary of the output transformer. This local feedback loop (6 db.) besides reducing the nonlinear distortion and improving the balance of the AC drive voltages, adjusts the phase characteristic of the driver and output stages at high frequencies so that the main loop of Unity Coupled Negative feedback (32 db.) can be applied around the entire amplifier to the grid of T, with complete stability.

From the plates of T,, T,, the signal is then fed through C and C u to the grids of T,, T, which is a balanced cathode follower. In the cathode circuits of T T there are four potentiometers on each side (0,, 0,. 0,, D and 0,. D,, 0,. and D bypassed by C and C With the arrangement used in the cathode circuits of T T it is possible to directly couple the cathode follower to the output tubes and at the same time, adjust the bias individually for each of the output tubes, for optimum balance of zero signal plate current.

The output tubes P,-P are operated as a parallel push-pull unity coupled, output stage, operating class AB. The resistors in the first grid of each output stage tube (R R R,,-,, R R R R R along with the inductors in the plate of each tube of the output stage (L through L,,), the resistors in the cathode of each tube of the output stage (R R R R R R R and R and the resistors in the screen grid of each tube in the output stage (R R R R R R R and R have been incorporated to stop oscillation of any individual tube due to the parallel configuration. The amount of current flowing through each tube in the output section is checked by measuring the voltage drop across the cathode resistor of each tube individually. By use of the switch S101 (Meter Range switch) and front panel meter M101, not illustrated, each tube can be individually measured, and by means of the potentiometers previously mentioned (D to D adjusted.

Five windings are filarly wound as the primary of the output transformer (including the winding for negative feedback). Sections of the primary and secondary are interwound to achieve maximal coupling, and balance between windings while keeping shunt capacitance low. Where coupling is particularly critical, such as between plate and cathode winding of the primary and the winding for the cathode follower driver, coupling is improved by the capacitance coupling of the windings in phase (C C C and C It is important to stress that the winding for the negative feedback be filarly wound with the rest of the primary windings but not capacitively coupled to any winding at the primary. The reason for this is that the voltage used as negative feedback should be in proportion to, and as similar as possible to the total current of the output stage. This technique of applying feedback, called Unity Coupled Negative Feedback, makes possible the use of up to 40 db. of total feedback with excellent stability, and nonlinear distortion is reduced to below 0.1 percent.

By means of the output impedance selector switch, not illustrated, it is possible to choose any one of six output impedances l, 4,8, 16, 50 or 64 ohms).

lclaim:

l. A power amplifier, including:

a first vacuum tube having a first anode, cathode, screen grid and control grid;

a second vacuum tube having a second anode, cathode,

screen grid, and control grid;

a transformer including a first primary winding having a predetermined number of turns, a center tap, and first and second ends;

a source of supply voltage connected to said center tap;

means connecting said first and second ends of said first primary winding to said first and second anodes, respectively;

a second primary winding of said transformer having said predetermined number of turns, a center tap, and first and second ends, said second primary winding being cofilarly wound with said first primary winding, the center tap of said second primary winding being DC connected to ground;

means connecting said first and second ends of said second primary winding to said first and second cathodes, respectively;

means for capacitively coupling said first and second ends.

of said first primary winding to said second and first ends, respectively, of said second primary winding to maintain the cathode of each of said vacuum tubes at the same AC potential as the anode of the other;

a third primary winding of said transformer having a center tap and first and second ends, the center tsp of said third primary winding being connected to a reference point;

means connecting said first and second ends of said third primary winding to said first and second screen grids, respectively, in such sense as to provide on the screen grid of each tube the same phase as exists on the cathode of that tube, said third primary winding being cofilarly wound with said first and second primary windings and having said predetermined number of turns; and

input means connected in driving relationship to said first and second control grids.

2. The combination according to claim 1, wherein said input means includes a drive amplifier for said power amplifier connected in driving relation to said first and second control grids;

a fourth primary winding of said transformer having said predetermined number of turns and cofilarly wound with said first primary winding;

means deriving external feedback voltage for said drive amplifier from said fourth primary winding;

a common core for all said primary windings; and

a secondary winding of said transformer wound on said core.

3. A power amplifier including:

a plurality of first vacuum tubes having each a first anode, cathode, screen grid and control grid, the same plurality of second vacuum tubes having each a second anode, cathode, screen grid and control grid;

means connecting all said first anodes to a common anod terminal;

means connecting all said second anodes to a conmion anode terminal;

a transformer, a first push-pull primary winding of said transformer connecting said common anode terminals;

means connecting all said first screen grids to a first common screen grid terminal; l

means connecting all said second screen grids to a second common screen grid terminal;

a second push-pull primary winding of said transformer cofilar with said first primary winding, having the same number of turns as said first primary winding, and connecting said common screen grid terminals;

means for maintaining the screen grid of each said tubes at the same AC voltage as its cathode;

means for driving all said first control grids in opposite phase to all said second control grids, said last means including first and second push-pull direct coupled cathode followers connected in driving relationship with said first and second plurality of vacuum tubes, respectively;

potentiometer bias means for pemritting individual adjustment of the biases of said control grids to provide the same no signal current for each vacuum tube;

capacitively bypassed cathode resistances for said cathode followers; and

means for providing bootstrap loading for said cathode followers, said last means including a third push-pull primary winding of said transformer filarly wound with and containing the same number of turns as said first primary winding and AC coupled to cathodes of said cathode followers.

4. The combination according to claim 3, wherein is further provided a fourth push-pull primary winding of said transformers connected between the cathodes of said tubes and a point of reference potential and arranged to maintain each of transformer filarly wound with and containing the same' number of turns as said first primary winding, providing unity coupled negative feedback for all said first and second output vacuum tubes.

6. An amplifier, including a vacuum tube having a plate, a control grid and a cathode, a load for said vacuum tube, comprising:

afirst primary winding of an output transformer;

a feedback circuit for said vacuum tube including a further primary winding of said output transformer cofilar with said first primary winding and having the same number of turns; and

means providing drive signal to said control grid, said last named means including said first primary winding, wherein said means providing drive signal is a vacuum tube having a cathode load, said cathode load comprising said first primary winding.

7. The combination according to claim 6, wherein said cathode load includes an AC' bypassed potentiometer resistance in series with said first primary winding, and wherein said control grid is driven from a variable tap on said potentiometer resistance.

8. A power amplifier, comprising:

amplifier means having an input circuit for receiving input signals to be amplified and an output circuit for providing amplified versions of said input signals;

a transformer including first and second cofilar primary windings;

input means including said first primary winding for applying said input signals to said input circuit;

' means for connecting said second primary winding in negative feedback relation between said output circuit and said input means, wherein said first and second primary windings have the same number of turns, and wherein said amplifier means includes at least one vacuum tube having an anode, a cathode, and a control grid, said input circuit including a variable resistor connected to said control grid, and wherein said input means includes a cathode follower for providing said input signals and an AC bypass capacitor connected across said variable resistor and in series between said cathode follower and v said first primary winding.

9. A power amplifier, comprising:

amplifier means having an input circuit for receiving input signals to be amplified and an output circuit for providing amplified versions of said input signals;

a transformer including first and second cofilar primary windings;

input means including said first primary winding for applying said input signals to said input circuit; and

means for connecting said second primary winding in negative feedback relation between said output circuit and said input means, wherein said first and second primary windings have the same number of turns, and wherein said amplifier means includes a plurality of vacuum tubes, each vacuum tube having an anode, a cathode and a control grid, said input circuit including a plurality of variable resistors, each variable resistor connected to a respective control grid, and wherein said input means includes a cathode follower for providing said input signals and an AC bypass capacitor connected in parallel across all of said variable resistors and in series between said cathode follower and said first primary winding.

10. A power amplifier circuit, comprising:

a first plurality of vacuum tubes and a second equal plurality of vacuum tubes, all of said vacuum tubes including an anode, a cathode, and a control grid;

means for AC coupling the anodes of said first plurality of vacuum tubes to the cathodes of said second plurality of vacuum tubes, and for AC coupling the anodes of said second plurality of vacuum tubes to the cathodes of said first plurality of vacuum tubes;

a transformer including first and second cofilar primary windings, said windings each having the same number of turns, a center tap, and first and second ends;

means for connecting the center taps of said first and second primary windings to respective DC reference potentials;

means for driving said first and second pluralities of vacuum tubes in push-pull relation, said means for driving including first and second cathode follower means for providing respective portions of a push-pull AC signal to be amplified, means for AC coupling said portions to respective 7 ends of said first primary winding, and means for connecting said first end of said first primary winding to the control grids of said first plurality of vacuum tubes and for connecting said second end of said first primary winding to the control grids of said second plurality of vacuum tubes; and

means for connecting said first and second ends of said second primary winding to said means for driving in negative feedback relation.

11. The circuit according to claim 10 wherein said means for AC coupling includes: third and fourth primary windings of said transformer, each cofilar with and having the same number of turns as said first and second primary windings, said third and fourth primary windings each having a center tap and first and second ends; means for connecting the anodes of said first plurality of vacuum tubes to said first end of said third primary winding and for connecting said anodes of said second plurality of vacuum tubes to the second end of said third primary winding; means for connecting the cathodes of said first plurality of vacuum tubes to the first end of said fourth primary winding and for connecting the cathodes of said second plurality of vacuum tubes to the second end of said fourth primary winding; means for capacitively coupling the first end of said third primary winding to the second end of said fourth primary winding; and means for capacitively coupling the second end of said third primary winding to the first end of said fourth primary winding 12. The circuit according to claim 1 1, wherein said vacuum tubes each includes a screen grid, and wherein said transformer further includes a fifth primary winding having the same number of turns as and cofilar with said first, second, third and fourth primary windings, said fifth primary winding having a center tap connected to reference potential, a first end connected to each screen grid in said first plurality of vacuum tubes, and a second end connected to each screen grid in said second plurality of vacuum tubes.

13. A power amplifier, including:

a plurality of first vacuum tubes having each a first anode,

cathode, screen grid and control grid, the same plurality of second vacuum tubes having each a second anode, cathode, screen grid and control grid;

means connecting all said first anodes to a common anode terminal;

means connecting all said second anodes to a common anode terminal;

a transformer;

a first push-pull primary winding of said transformer connecting said anode terminals;

means connecting all said first screen grids to a first common screen grid terminal;

means connecting all said second screen grids to a second common screen grid terminal;

a second push-pull primary winding of said transformer cofilar with and having the same number of turns as said first primary winding and connecting said first and second common screen grid terminals;

means including said second primary winding for maintaining the screen grid of each of said tubes at the same AC voltage as its anode;

means for driving all said first control grids in opposite phase to all said second control grids, said last means including push-pull connected drive vacuum tubes, potentiometer bias circuits for individually adjusting the biases of said control grids to provide the same peak voltage at all said screen grids, and capacitively bypassed cathode resistances for said drive vacuum tubes; and

means providing negative feedback for all said first and second vacuum tubes, said last means including a feedback pushull rimary winding of said transformer tilarly wound wit an contamrng the same number of turns as said first primary winding and connected to said control grids.

14. The combination according to claim 13, wherein is further provided a further push-pull primary winding connected between said cathodes and a point of reference potential, means including said further primary winding for maintaining each of the cathodes of said plurality of first vacuum tubes at the same AC potential as the anodes of the plurality of second vacuum tubes and the cathodes of the plurality of second vacuum tubes at the same AC potential as the anodes of the plurality of first vacuum tubes. 

1. A power amplifier, including: a first vacuum tube having a first anode, cathode, screen grid and control grid; a second vacuum tube having a second anode, cathode, screen grid, and control grid; a transformer including a first primary winding having a predetermined number of turns, a center tap, and first and second ends; a source of supply voltage connected to said center tap; means connecting said first and second ends of said first primary winding to said first and second anodes, respectively; a second primary winding of said transformer having said predetermined number of turns, a center tap, and first and second ends, said second primary winding being cofilarly wound with said first primary winding, the center tap of said second primary winding being DC connected to ground; means connecting said first and second ends of said second primary winding to said first and second cathodes, respectively; means for capacitively coupling said first and second ends of said first primary winding to said second and first ends, respectively, of said second primary winding to maintain the cathode of each of said vacuum tubes at the same AC potential as the anode of the other; a third primary winding of said transformer having a center tap and first and second ends, the center tap of said third primary winding being connected to a reference point; means connecting said first and second ends of said third primary winding to said first and second screen grids, respectively, in such sense as to provide on the screen grid of each tube the same phase as exists on the cathode of that tube, said third primary winding being cofilarly wound with said first and second primary windings and having said predetermined number of turns; and input means connected in driving relationship to said first and second control grids.
 2. The combinaTion according to claim 1, wherein said input means includes a drive amplifier for said power amplifier connected in driving relation to said first and second control grids; a fourth primary winding of said transformer having said predetermined number of turns and cofilarly wound with said first primary winding; means deriving external feedback voltage for said drive amplifier from said fourth primary winding; a common core for all said primary windings; and a secondary winding of said transformer wound on said core.
 3. A power amplifier including: a plurality of first vacuum tubes having each a first anode, cathode, screen grid and control grid, the same plurality of second vacuum tubes having each a second anode, cathode, screen grid and control grid; means connecting all said first anodes to a common anode terminal; means connecting all said second anodes to a common anode terminal; a transformer, a first push-pull primary winding of said transformer connecting said common anode terminals; means connecting all said first screen grids to a first common screen grid terminal; means connecting all said second screen grids to a second common screen grid terminal; a second push-pull primary winding of said transformer cofilar with said first primary winding, having the same number of turns as said first primary winding, and connecting said common screen grid terminals; means for maintaining the screen grid of each said tubes at the same AC voltage as its cathode; means for driving all said first control grids in opposite phase to all said second control grids, said last means including first and second push-pull direct coupled cathode followers connected in driving relationship with said first and second plurality of vacuum tubes, respectively; potentiometer bias means for permitting individual adjustment of the biases of said control grids to provide the same no signal current for each vacuum tube; capacitively bypassed cathode resistances for said cathode followers; and means for providing bootstrap loading for said cathode followers, said last means including a third push-pull primary winding of said transformer filarly wound with and containing the same number of turns as said first primary winding and AC coupled to cathodes of said cathode followers.
 4. The combination according to claim 3, wherein is further provided a fourth push-pull primary winding of said transformers connected between the cathodes of said tubes and a point of reference potential and arranged to maintain each of the cathodes of said plurality of first vacuum tubes at the same AC potential as the anodes of said plurality of second vacuum tubes and to maintain the cathodes of said plurality of second vacuum tubes at the same AC potential as the anodes of said plurality of first vacuum tubes.
 5. The combination according to claim 4, wherein is further provided a fifth feedback push-pull primary winding of said transformer filarly wound with and containing the same number of turns as said first primary winding, providing unity coupled negative feedback for all said first and second output vacuum tubes.
 6. An amplifier, including a vacuum tube having a plate, a control grid and a cathode, a load for said vacuum tube, comprising: a first primary winding of an output transformer; a feedback circuit for said vacuum tube including a further primary winding of said output transformer cofilar with said first primary winding and having the same number of turns; and means providing drive signal to said control grid, said last named means including said first primary winding, wherein said means providing drive signal is a vacuum tube having a cathode load, said cathode load comprising said first primary winding.
 7. The combination according to claim 6, wherein said cathode load includes an AC bypassed potentiometer resistance in series with said first primary winding, and wherein saiD control grid is driven from a variable tap on said potentiometer resistance.
 8. A power amplifier, comprising: amplifier means having an input circuit for receiving input signals to be amplified and an output circuit for providing amplified versions of said input signals; a transformer including first and second cofilar primary windings; input means including said first primary winding for applying said input signals to said input circuit; means for connecting said second primary winding in negative feedback relation between said output circuit and said input means, wherein said first and second primary windings have the same number of turns, and wherein said amplifier means includes at least one vacuum tube having an anode, a cathode, and a control grid, said input circuit including a variable resistor connected to said control grid, and wherein said input means includes a cathode follower for providing said input signals and an AC bypass capacitor connected across said variable resistor and in series between said cathode follower and said first primary winding.
 9. A power amplifier, comprising: amplifier means having an input circuit for receiving input signals to be amplified and an output circuit for providing amplified versions of said input signals; a transformer including first and second cofilar primary windings; input means including said first primary winding for applying said input signals to said input circuit; and means for connecting said second primary winding in negative feedback relation between said output circuit and said input means, wherein said first and second primary windings have the same number of turns, and wherein said amplifier means includes a plurality of vacuum tubes, each vacuum tube having an anode, a cathode and a control grid, said input circuit including a plurality of variable resistors, each variable resistor connected to a respective control grid, and wherein said input means includes a cathode follower for providing said input signals and an AC bypass capacitor connected in parallel across all of said variable resistors and in series between said cathode follower and said first primary winding.
 10. A power amplifier circuit, comprising: a first plurality of vacuum tubes and a second equal plurality of vacuum tubes, all of said vacuum tubes including an anode, a cathode, and a control grid; means for AC coupling the anodes of said first plurality of vacuum tubes to the cathodes of said second plurality of vacuum tubes, and for AC coupling the anodes of said second plurality of vacuum tubes to the cathodes of said first plurality of vacuum tubes; a transformer including first and second cofilar primary windings, said windings each having the same number of turns, a center tap, and first and second ends; means for connecting the center taps of said first and second primary windings to respective DC reference potentials; means for driving said first and second pluralities of vacuum tubes in push-pull relation, said means for driving including first and second cathode follower means for providing respective portions of a push-pull AC signal to be amplified, means for AC coupling said portions to respective ends of said first primary winding, and means for connecting said first end of said first primary winding to the control grids of said first plurality of vacuum tubes and for connecting said second end of said first primary winding to the control grids of said second plurality of vacuum tubes; and means for connecting said first and second ends of said second primary winding to said means for driving in negative feedback relation.
 11. The circuit according to claim 10 wherein said means for AC coupling includes: third and fourth primary windings of said transformer, each cofilar with and having the same number of turns as said first and second primary windings, said third and fourth primary windings each having a center tap and firsT and second ends; means for connecting the anodes of said first plurality of vacuum tubes to said first end of said third primary winding and for connecting said anodes of said second plurality of vacuum tubes to the second end of said third primary winding; means for connecting the cathodes of said first plurality of vacuum tubes to the first end of said fourth primary winding and for connecting the cathodes of said second plurality of vacuum tubes to the second end of said fourth primary winding; means for capacitively coupling the first end of said third primary winding to the second end of said fourth primary winding; and means for capacitively coupling the second end of said third primary winding to the first end of said fourth primary winding.
 12. The circuit according to claim 11, wherein said vacuum tubes each includes a screen grid, and wherein said transformer further includes a fifth primary winding having the same number of turns as and cofilar with said first, second, third and fourth primary windings, said fifth primary winding having a center tap connected to reference potential, a first end connected to each screen grid in said first plurality of vacuum tubes, and a second end connected to each screen grid in said second plurality of vacuum tubes.
 13. A power amplifier, including: a plurality of first vacuum tubes having each a first anode, cathode, screen grid and control grid, the same plurality of second vacuum tubes having each a second anode, cathode, screen grid and control grid; means connecting all said first anodes to a common anode terminal; means connecting all said second anodes to a common anode terminal; a transformer; a first push-pull primary winding of said transformer connecting said anode terminals; means connecting all said first screen grids to a first common screen grid terminal; means connecting all said second screen grids to a second common screen grid terminal; a second push-pull primary winding of said transformer cofilar with and having the same number of turns as said first primary winding and connecting said first and second common screen grid terminals; means including said second primary winding for maintaining the screen grid of each of said tubes at the same AC voltage as its anode; means for driving all said first control grids in opposite phase to all said second control grids, said last means including push-pull connected drive vacuum tubes, potentiometer bias circuits for individually adjusting the biases of said control grids to provide the same peak voltage at all said screen grids, and capacitively bypassed cathode resistances for said drive vacuum tubes; and means providing negative feedback for all said first and second vacuum tubes, said last means including a feedback push-pull primary winding of said transformer filarly wound with and containing the same number of turns as said first primary winding and connected to said control grids.
 14. The combination according to claim 13, wherein is further provided a further push-pull primary winding connected between said cathodes and a point of reference potential, means including said further primary winding for maintaining each of the cathodes of said plurality of first vacuum tubes at the same AC potential as the anodes of the plurality of second vacuum tubes and the cathodes of the plurality of second vacuum tubes at the same AC potential as the anodes of the plurality of first vacuum tubes. 